U.S. patent number 5,015,617 [Application Number 07/336,700] was granted by the patent office on 1991-05-14 for catalyst for purifying exhaust gas and method for production thereof.
This patent grant is currently assigned to Nippon Shokubai Kagaku Kogyo Co., Ltd.. Invention is credited to Shinya Kitaguchi, Tomohisa Ohata, Kazuo Tsuchitani.
United States Patent |
5,015,617 |
Ohata , et al. |
May 14, 1991 |
**Please see images for:
( Certificate of Correction ) ** |
Catalyst for purifying exhaust gas and method for production
thereof
Abstract
A catalyst for purifying exhaust gas produced by coating
honeycomb carriers of monolithic structure with a catalyst
composition comprising cerium oxide particles stabilized with (a)
zirconia or (b) zirconia and at least one member selected from the
group consisting of yttria and calcia, refractory inorganic oxide
particles, 0.02 to 2% by weight of rhodium based on said catalyst
composition, and 0 to 10% by weight of at least one platinum-family
metal selected from the group consisting of platinum and palladium
based on said catalyst composition.
Inventors: |
Ohata; Tomohisa (Himeji,
JP), Tsuchitani; Kazuo (Himeji, JP),
Kitaguchi; Shinya (Himeji, JP) |
Assignee: |
Nippon Shokubai Kagaku Kogyo Co.,
Ltd. (Osaka, JP)
|
Family
ID: |
26431806 |
Appl.
No.: |
07/336,700 |
Filed: |
April 12, 1989 |
Foreign Application Priority Data
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Apr 14, 1988 [JP] |
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63-90310 |
Apr 14, 1988 [JP] |
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63-90311 |
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Current U.S.
Class: |
502/304;
502/349 |
Current CPC
Class: |
B01J
23/63 (20130101); B01J 23/58 (20130101); B01D
53/945 (20130101); B01D 2255/20715 (20130101); Y02T
10/12 (20130101); B01D 2255/40 (20130101); B01D
2255/206 (20130101) |
Current International
Class: |
B01J
23/63 (20060101); B01J 23/54 (20060101); B01J
23/58 (20060101); B01D 53/94 (20060101); B01J
021/06 () |
Field of
Search: |
;502/304,349 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0170588 |
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Feb 1986 |
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EP |
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0171151 |
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Feb 1986 |
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EP |
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0329302 |
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Aug 1989 |
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EP |
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2595265 |
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Sep 1987 |
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FR |
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0119891 |
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Nov 1974 |
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JP |
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0124442 |
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Sep 1981 |
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JP |
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57-153737 |
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1982 |
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JP |
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57-822382 |
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1982 |
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JP |
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5279215 |
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1982 |
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JP |
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57-24638 |
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Feb 1982 |
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JP |
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58-143838 |
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Aug 1983 |
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JP |
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60-171223 |
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1985 |
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JP |
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61-11147 |
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1986 |
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JP |
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61-157347 |
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Jul 1986 |
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JP |
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62-168544 |
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Jul 1987 |
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JP |
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63-23739 |
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Feb 1988 |
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JP |
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63-116741 |
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May 1988 |
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JP |
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Other References
Yao, H. C., "Surface Interactions in the System Rh/Al.sub.2 O.sub.3
", Journal of Catalysis, 50, pp. 407-418 (1977)..
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Primary Examiner: Garvin; Patrick P.
Assistant Examiner: Fourson; George R.
Attorney, Agent or Firm: Behr; Omri M.
Claims
What is claimed is:
1. A catalyst for purifying exhaust gas produced by coating
honeycomb carriers of monolithic structure with a catalyst
composition comprising [10- 50% by weight of ] (1) cerium oxide
particles dispersed on (a) zirconia or (b) zirconia and at least
one member selected from the group consisting of yttria and calcia,
said dispersed particles being obtained by impregnating zirconium
oxide or hydroxide having a specific surface area of from 60-200
mg. with a solution of (i) a cerium salt or (ii) a cerium salt and
at least one salt selected from the group consisting of yttrium
salts and calcium salts, drying and then calcining the resultant
impregnated particles, wherein said cerium oxide is contained in a
range of 10 to 50% by weight based on said zirconia and the chart
of the x-ray diffraction spectrum thereof does not show a peak for
the crystalline form of cerium oxide, (2) refractory inorganic
oxide particles, (3) 0.02 to 2% by weight of rhodium and (4) 0 to
10% by weight of at least one platinum-family metal selected from
the group consisting of platinum and palladium, based on said
catalyst composition.
2. A catalyst according to claim 1, wherein the chart of the x-ray
diffraction spectrum of said impregnated particles after 10 hours
calcination in air at a temperature of 900.degree. C. shows a peak
for the tetragonal crystal of zirconia, and no peak for cerium
oxide, and said cerium oxide is incorporated substantially in the
form of solid solution with (a) zirconia or (b) zirconia and at
least one member selected from the group consisting of yttria and
calcia.
3. A catalyst according to claim 1, wherein said catalyst
composition contains said impregnated particles in the range of 10
to 80% by weight.
4. A catalyst according to claim 1, wherein said rhodium is carried
on at least one type of particle selected from the group consisting
of said impregnated particles and said refractory inorganic oxide
particles.
5. A catalyst according to claim 4, wherein said rhodium is carried
in an amount in the range of 1 to 20% by weight based on the weight
of said refractory inorganic oxide.
6. A catalyst according to claim 4, wherein said rhodium carrying
refractory inorganic oxide is contained in said catalyst
composition in the form of aggregate particles having an average
diameter in the range of 0.5 to 20 .mu.m.
7. A catalyst according to claim 1, wherein said refractory
inorganic oxide comprises at least one member selected from the
group consisting of activated alumina, silica, titania, and
zirconia.
8. A catalyst according to claim 7, wherein said refractory
inorganic oxide comprises activated alumina.
9. A catalyst according to claim 8, wherein said activated alumina
further contains at least one oxide selected from the group
consisting of cerium, yttrium, lanthanum, neodymium, zirconium,
iron, barium, nickel, and cobalt oxides.
10. A catalyst according to claim 1, wherein said dispersed cerium
oxide is dispersed on zirconia.
11. A catalyst according to claim 1, wherein the amount of said
platinum-family metal selected from the group consisting of
platinum and palladium is in the range of 0.001 to 5% by weight
based on the weight of said catalyst composition.
12. A catalyst according to claim 1, wherein said catalyst
composition comprises refractory inorganic oxide particles carrying
no rhodium.
13. A catalyst according to claim 12, wherein said platinum-family
metal selected from the group consisting of platinum and palladium
is carried on at least one member selected from the group
consisting of said rhodium-carrying refractory inorganic oxide,
said impregnated particles, and said refractory inorganic oxide
carrying no rhodium.
14. A catalyst according to claim 12, wherein said refractory
inorganic oxide carrying no rhodium comprises at least one member
selected from the group consisting of activated alumina and
silica.
15. A catalyst according to claim 14, wherein said refractory
inorganic oxide comprises activated alumina.
16. A catalyst according to claim 15, wherein said activated
alumina further contains the oxide of at least one member selected
from the group consisting of cerium, yttrium, lanthanum, neodymium,
zirconium, iron, barium, nickel, and cobalt oxides.
17. A catalyst according to claim 1, wherein said platinum-family
metal selected from the group consisting of platinum and palladium
is carried on at least one member selected from the group
consisting of said rhodium-carrying refractory inorganic oxide and
said impregnated particles.
18. A catalyst according to claim 1, wherein the amount of said
catalyst composition carried is in the range of 50 to 300 g per
liter of said carrier.
19. A catalyst according to claim 1, wherein said rhodium and said
platinum metal selected from the group consisting of platinum and
palladium are carried on at least one member selected from the
group consisting of said impregnated particles and said refractory
inorganic oxide particles.
20. A catalyst according to claim 19, wherein said catalyst
composition further comprises refractory inorganic oxide particles
carrying no rhodium.
21. A catalyst according to claim 20, wherein said refractory
inorganic oxide comprises activated alumina.
22. A catalyst according to claim 21 wherein said refractory
inorganic oxide further comprises at least one oxide selected from
the group of oxides consisting of cerium, yttrium, lanthanum,
neodymium, zirconium, iron, barium, nickel and cobalt oxides.
23. A catalyst according to claim 1, wherein said cerium oxide
particles are contained in a concentration range of (a) 10 to 50%
by weight as cerium oxide or the combination of said cerium oxide
particles and yttria or calcia are contained in a concentration
range of (b) 10 to 50% by weight as cerium oxide and 0.5 to 15% by
weight as yttria or calcia, based on the amount of zirconia, and
the chart of the x-ray diffraction spectrum analysis thereof does
not show a peak for the crystalline form of cerium oxide.
24. A catalyst according to claim 1, wherein said rhodium is
carried on said refractory inorganic oxide particles.
25. A method for the production of a catalyst according to claim 1
which comprises preparing a catalyst composition composed of a
refractory inorganic oxide having rhodium carried thereon in an
amount in the range of 0.02 to 2% by weight based on the amount of
said catalyst composition, cerium oxide particles dispersed on (a)
zirconia or (b) zirconia and at least one member selected from the
group consisting of yttria and calcia, and refractory inorganic
oxide particles having carried thereon 0 to 10% by weight, based on
the amount of said catalyst composition, of at least one
platinum-family metal selected from the group consisting of
platinum and palladium, converting said catalyst composition into
an aqueous slurry, coating honeycomb carriers of monolithic
structure with said aqueous slurry, and subsequently drying the
resultant coated carrier to obtain the catalyst.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a catalyst for purifying exhaust gas and
a method for production thereof. More particularly, it relates to a
catalyst for purifying exhaust gas by simultaneous removal of such
harmful components as hydrocarbons (HC), carbon monoxide (CO), and
nitrogen oxides (NO.sub.x) contained in the exhaust gas from the
internal combustion engine of an automobile, for example,
especially a catalyst for purifying exhaust gas which retains
outstanding durability even when it is used under such harsh
conditions as experienced in an oxidizing atmosphere at elevated
temperatures and which exhibits a high scrubbing performance at low
temperatures to the harmful components.
2. Description of the Prior Art
Heretofore, there have been proposed numerous catalysts for
purifying exhaust gas emanating from the internal combustion engine
of an automobile, for example. At present, three-way catalysts for
simultaneous removal of CO, HC, and NO.sub.x constitute the
mainstream of the trend of the catalyst for purifying exhaust
gas.
The three-way catalysts mentioned above are generally produced by
depositing such active components as platinum (Pt), palladium (Pd),
and rhodium (Rh) either singly or jointly as dispersed on such a
refractory inorganic oxide as alumina or zirconia in a powdered
form thereby preparing a catalyst composition and coating a
monolithic carrier of cordierite, for example, with the catalyst
composition. Of these three-way catalysts, that which has
platinum-family metals including rhodium dispersed densely on
activated alumina and also has cerium oxide (ceria incorporated in
the catalyst composition is claimed to possess an outstanding
purifying ability.
Further, the practice of adding to the catalyst active composition
a promoter component selected from among the compounds of rare
earth elements, the compounds of the elements of Group VIII, the
alkali metal compounds, and the alkaline earth metal compounds for
the purpose of promoting and stabilizing the catalytic activity of
noble metals as the active component of catalyst is now in vogue.
Particularly, cerium oxide (CeO.sub.2), a rare earth compound,
manifests a conspicuous effect as a promoter catalyst and has found
widespread utility
Generally, rhodium is a noble metal component which contributes in
a large measure to the catalyst's ability to purify the waste gas
of NO.sub.x and permit combustion at low temperatures. It has been
known, however, that under such harsh conditions as experienced in
an oxidizing atmosphere at elevated temperatures, rhodium is
diffused in an alumina and consequently deprived of activity [refer
to J. Catal., 50, 407 (1977)] and rhodium and ceria undergo
undesirable interaction. In Japanese Patent Laid-open SHO
61(1986)-11,147, for example, it is disclosed that the interaction
of rhodium and alumina is diminished by regulating the particle
diameter of rhodium. This method, however, necessitates intricate
steps as for wet reduction. We have confirmed that this method is
not sufficient for the purpose of curbing the interaction of
rhodium and ceria. As ways of incorporating ceria in the catalyst
composition, a method which comprises depositing ceria on the
catalyst composition by impregnating activated alumina with a
solution of ceria and a method effects the incorporation by pouring
the bulk of an oxide, hydroxide, or carbonate of cerium into the
other components of the catalyst composition have been known. When
the ceria is deposited as on activated alumina, however, a
disadvantage ensues that the ceria particles are liable to grow and
the ceria is deficient in thermal stability. When the bulk of
cerium, compound is poured into the catalyst composition, there is
a disadvantage that the consumption of ceria is so large as to
render the produced catalyst very expensive and the frequency of
contact between ceria and rhodium is increased to an extent of
inducing an undesirable interaction and consequent loss of
activity. Japanese Patent Laid-Open SHO 60(1985)-171,223 and
Japanese Patent Laid-Open SHO 62(1987)-168,544 have a disclosure to
the effect that all the rare earth elements are as effective in
stabilizing zirconium as yttrium, calcium, and magnesia. The effect
which such rare earth elements used in a catalyst are to manifest
in aiding in the catalysis remains yet to be clarified.
In the field of catalysts, zirconia is used mostly for the purpose
of stabilizing the specific surface area and other similar physical
properties of the catalyst composition. As concerns the use of
zirconia as a carrier for a platinum-family metal, various methods
are proposed as in Japanese Patent Publication SHO 57(1982)-29,215
and Japanese Patent Laid-Open SHO 57(1982)-153,737, for
example.
Zirconia possesses highly desirable properties as a carrier for a
platinum-family metal. It nevertheless is highly susceptible of
thermal deterioration and liable to undergo heavy loss of activity
during a protracted use at elevated temperatures. An invention
which consists in using zirconia as a thermally stable sintered
article is disclosed, for example, in Japanese Patent Publication
SHO 57(1982)-2,382. The thermally stable sintered article of
zirconia is deficient in binding force with such catalyst
components as platinum-family metal and ceria and liable to induce
growth of particles of the active component and growth of crystals
and, as a result, the produced catalyst suffers from low thermal
stability and poor durability.
An object of this invention, therefore, is to provide a novel
catalyst for purifying exhaust gas and a method for the production
thereof.
Another object of this invention is to provide a catalyst for
purifying exhaust gas which retains outstanding durability when it
is used under such harsh conditions as encountered in an oxidizing
atmosphere at elevated temperatures and exhibits a high scrubbing
ability at low temperatures to the harmful components contained in
the exhaust gas.
SUMMARY OF THE INVENTION
The objects of the invention described above are accomplished by a
catalyst for purifying exhaust gas produced by coating honeycomb
carriers of monolithic structure with a catalyst composition
comprising cerium oxide particles stabilized with (a) zirconia or
(b) zirconia and at least one member selected from the group
consisting of yttria and calcia, refractory inorganic oxide
particles, 0.02 to 2% by weight, based on the amount of catalyst
composition, of rhodium, and 0 to 10% by weight, based on the
amount of catalyst composition, of at least one platinum-family
metal selected from the group consisting of platinum and
palladium.
In the present invention, it has been found that the durability
characteristic of rhodium is prominently improved by allowing
rhodium to be incorporated in the catalyst composition as deposited
in a high concentration on particles of such a refractory inorganic
oxide as alumina, and adjusted in the form of aggregate particles
of a relatively large average diameter.
It has been further found that an amorphous ceria compound
excellent in thermal stability and promoting effect is obtained by
depositing ceria as dispersed on zirconia or zirconium hydroxide of
a high surface area and this amorphous ceria hardly undergoes an
interaction with rhodium. It has been also found that the catalyst
composition which contains the inorganic oxide carrying rhodium in
a high concentration obtained as described above and the ceria
stabilized with zirconia inhibits the undesirable interaction with
rhodium and retains outstanding durability and low-temperature
activity even when it is used under such harsh conditions as
encountered in an oxidizing atmosphere at elevated temperatures.
This invention has been completed as the result.
The objects described above are also accomplished by a method for
the production of a catalyst for purifying exhaust gas, which
comprises preparing a catalyst composition composed of a refractory
inorganic oxide having rhodium carried therein in an amount in the
range of 0.02 to 2 % by weight based on the amount of the catalyst
composition, cerium oxide particles stabilized with (a) zirconia or
(b) zirconia and at least one member selected from the group
consisting of yttria and calsia, and refractory inorganic oxide
particles having carried thereon 0 to 10% by weight, based on the
amount of the catalyst composition, of at least one platinum-family
metal selected from the group consisting of platinum and palladium,
converting the catalyst composition into an aqueous slurry, coating
honeycomb carriers of monolithic structure with the aqueous slurry,
and subsequently drying the resultant coated carrier.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an X-ray diffraction diagram of a typical catalyst
according to this invention,
FIGS. 2 to 4 are X-ray diffraction diagrams of catalysts for
comparison, and
FIG. 5 is a chart showing the results of determination of the bond
energy of Ce 3d 5/2 as measured of the ceria used in the catalyst
of this invention and in the catalysts for comparison with an X-ray
photoelectric spectroscope.
EXPLANATION OF THE PREFERRED EMBODIMENT
The catalyst composition to be used in the present invention
comprises cerium oxide stabilized with (a) zirconia or (b) zirconia
and at least one member selected from the group consisting of
yttria and calcia, refractory inorganic oxide particles, rhodium
and optionally at least one platinum-family metal selected from the
group consisting of platinum and palladium. In this catalyst
composition, the rhodium is deposited either alone or in
combination with the platinum-family metal on the stabilized cerium
oxide particles and/or the refractory inorganic oxide
particles.
The amount of rhodium to be carried is in the range of 0.02 to 2%
by weight, preferably 0.1 to 1 % by weight, based on the amount of
the catalyst composition. Preferable method for supporting rhodium
resides in supporting in an amount of 1-20 % by weight of high
supporting concentration to the refractory inorganic oxide
particles. If this amount is less than 1% by weight, the deposited
rhodium assumes a state approximating to that of ordinary high
dispersion, undergoes a heavy interaction with the refractory
inorganic oxide such as alumina, and fails to retain desired
durability. If this amount exceeds 20% by weight, the initial
performance is low and the rhodium particles are liable to grow
during the course of use. When the composition of the refractory
inorganic oxide supported rhodium is prepared in the form of
aggregate particles having a relatively large average diameter of
0.5 to 20 .mu.m, preferably 1 to 10 .mu.m and then dispersed in the
catalyst composition, it serves to mitigate the interaction or
reaction of rhodium with the refractory inorganic oxide or promoter
substance without impairing the efficiency of the reaction for
exhaust gas scrubbing.
The refractory inorganic oxides which are usable herein include
alumina, silica, titania, and zirconia, for example. It is
particularly desirable to use alumina, especially activated alumina
in the crystalline form of .gamma., .delta., or .theta..
The method for the deposition of the platinum-family metal
including rhodium is not specifically defined. The conventional
method of impregnation may be used for the deposition, for example.
The composition can be obtained by thoroughly mixing the aqueous
solution of a rhodium salt such as rhodium nitrate, rhodium
chloride, or rhodium sulfate with the refractory inorganic oxide in
the form of particles and then drying and calcining the resultant
mixture. The calcination temperature is in the range of 300.degree.
to 700.degree. C., preferably 400.degree. to 600.degree. C.
The cerium oxide composition stabilized with zirconia which
constitutes the second characteristic of the present invention is
obtained by impregnating zirconia or an amorphous zirconium hydrate
such as zirconium hydroxide in the form of minute particles of a
large surface area in the range of 60 to 200 m.sup.2 /g, preferably
80 to 200 m.sup.2 /g, with the aqueous solution of a cerium salt
and then drying the impregnated particles and calcing them at a
temperature in the range of 400.degree. to 700.degree. C.,
preferably 500.degree. to 600.degree. C. The cerium salts which are
usable herein include cerium nitrate, cerium sulfate, cerium
chloride, and cerium oxalate, for example.
The content of ceria in the cerium oxide composition stabilized
with zirconia is desired to be in the range of 10 to 50% by weight,
preferably 20 to 30% by weight. When the ceria content is in the
preferred range of 20 to 30% by weight, the produced composition
has ceria uniformly formed solid solutions.
The powdered zirconia-stabilized cerium oxide composition may
otherwise be produced by causing coprecipitation of the cerium salt
with a zirconium salt.
The powdered zirconia-stabilized cerium oxide composition produced
as described above, on analysis by X-ray diffraction, shows a
crystalline structure solely of zirconia. When this powder is
calcined in the air at 900.degree. C. for 10 hours, it shows in the
X-ray diffraction diagram a curve for the teragonal crystal of
zirconia, indicating that the zirconia is stabilized in the
tetragonal crystal by the ceria, as compared with the zirconia
powder containing no ceria and showing a peak for the monoclinic
crystal. The calcination is preferable to be carried preferably out
at a temperature in the range of 400.degree. to 700.degree. C.
Further, the specific surface area to be measured after 10 hours'
calcination in the air at 900.degree. C. is not more than 10
m.sup.2 /g in the case of the zirconia powder containing no ceria
and not less than 30 m.sup.2 /g in the care of the powdered
zirconia-stabilized cerium oxide composition containing ceria in
the range of 10 to 50 % by weight. This comparison clearly
indicates that the incorporation of ceria serves to stabilize
thermally the composition.
When a sintered article of zirconia which has acquired a specific
surface area of less than 60 m.sup.2 /g as the result of heat
treatment, for example, is used as a zirconia source, no solid
solution is obtained as evinced by the fact that the X-ray
diffraction shows crystalline structures for both ceria and
zirconia.
If the ceria content in the powdered zirconia-stabilized cerium
oxide composition is less than 10% by weight based on the amount of
zirconia, a peak for the monoclinic crystal of zirconia partly
appears after 10 hours' calcination at 950.degree. C. If the ceria
content exceeds 50% by weight, a peak for ceria appears.
The powdered zirconia-stabilized cerium oxide composition is
enabled to bring about a still better effect when the
zirconia-stabilized cerium oxide is further stabilized with yttria
and/or calcia. The content of yttria and/or calcia in the cerium
oxide composition which has been stabilized with zirconia and
yttria and/or calcia is in the range of 0.5 to 15% by weight,
preferably 1 to 5 % by weight. The stabilized cerium oxide
composition is identified by the fact that the X-ray diffraction
diagram of the composition shows no peak for the crystal of ceria.
Stabilization with yttria and/or calcia may be carried out by
impregnating a yttrium and/or calcium salt to zirconia or zirconium
hydrate during or after impregnating a cerium salt to zirconia or
zirconium hydrate, drying and calcining. The calcination
temperature is 400.degree. to 700.degree. C., preferably
500.degree. C. to 600.degree. C.
The stabilized cerium oxide of this invention is such that even
after it has been exposed to an oxidizing atmosphere at an elevated
temperature of not less than 800.degree. C., the X-ray diffraction
diagram thereof does not show a peak for the crystal of ceria or
yttria but shows mainly a peak for the tetragonal crystal of
zirconia. The absence of the peak for the crystal of ceria or
yttria from the X-ray diffraction diagram may be logically
explained by a supposition that cerium and yttrium both form a
solid solution with zirconia or they are deposited as idealy
dispersed in the interstices between the zirconia particles.
It has been confirmed by us that the yttrium and calcium
incorporated in the composition are effective in stabilizing the
zirconia and, at the same time, in altering the condition of
oxidation of ceria. The result of the test by the method of X-ray
photoelectric spectroscopy shows that the incorporation of yttrium
or calcium brings about an addition to the oxidation number of Ce.
This fact suggests that active oxygen seeds have been formed on the
surface of ceria.
The stabilized cerium oxide thus obtained is contained in a
concentration in the range of 10 to 80% by weight, preferably 20 to
40% by weight, based on the total amount of the catalyst
composition. The incorporation of the stabilized cerium oxide in
combination with the platinum-family metal permits production of a
highly satisfactory exhaust gas scrubbing catalyst for use in the
internal combustion engine.
The rhodium is carried on the particles of the cerium oxide
composition stabilized with zirconia alone or with zirconia in
combination with yttria or calcia and/or the particles of
refractory inorganic oxide as described above. Optionally,
particles of a refractory inorganic oxide carrying no rhodium may
be additionally incorporated in the catalyst composition. When a
refractory inorganic oxide such as activated alumina or silica
prepared with a large surface area is incorporated in the catalyst
composition, it enables the catalyst composition to acquire a
physical effect such as, for example, diffusion of gas. When the
refractory inorganic oxide has a rare earth element, a base metal
element such as iron, chromium, manganese, or nickel, or a
platinum-family metal other than rhodium carried thereon, the
produced catalyst composition enjoys a further improvement in the
purifying ability.
The platinum-family metal other than rhodium, when necessary, may
be deposited on the refractory inorganic oxide carrying rhodium,
the cerium oxide stabilized solely by zirconia or jointly by
zirconia and yttria and/or calcia, or the refractory inorganic
oxide not carrying rhodium. The amount of the platinum-family metal
thus deposited is in the range of 0 to 10% by weight, more
desirably 0.001 to 5% by weight, and most desirably 0.5 to 2% by
weight. This deposition is effected in the same manner as that of
rhodium mentioned above.
From the compositions obtained as described above, a finished
catalyst is produced by converting these compositions into an
aqueous slurry by the use of a ball mill, wash coating honeycomb
carriers of monolithic structure, for example, with the aqueous
slurry, then drying the coated honeycomb carriers, and optionally
calcining the dry coated honeycomb carriers. The calcination
temperature is in the range of 100.degree. to 700.degree. C.,
preferably 150.degree. to 500.degree. C.
The honeycomb carriers of monolithic structure which are usable in
this invention include carriers of such ceramic substances as
cordierite, mullite, and .alpha.-alumina and metallic monolithic
carriers made of stainless steel and Fe--Cr--Al alloy, for example.
The amount of the catalyst composition to be carried per liter of
these carriers is in the range of 50 to 300 g, preferably 100 to
250 g.
Now, the present invention will be described more specifically
below with reference to working examples. It should be noted,
however, that this invention is not limited to these examples.
EXAMPLE 1
With 100 ml of an aqueous solution of 50 g of cerium nitrate
[Ce(NO.sub.3).sub.3. 6H.sub.2 O], 50 g of minute particles of
zirconia having a specific surface area of 92 m.sup.2 /g (produced
by Daiichi Kigenso K.K.) were impregnated and mixed. Then the
impregnated minute particles were thoroughly dried and calcined at
500.degree. C. for one hour. (The product was labeled as "sample
No. 1.")
Control 1
Sample a was obtained by following the procedure of Example 1,
except that alumina .gamma.-alumina) having a specific surface area
of 155 m.sup.2 /g was used in place of zirconia.
Control 2
Sample b was obtained by following the procedure of Example 1,
except that zirconia having a specific surface area of 45 m.sup.2
/g was used instead.
EXAMPLE 2
Sample No. 2 was obtained by following the procedure of Example 1,
except that the amount of cerium nitrate was changed to 100 g.
Control 3
Sample c was obtained by following the procedure of Example 1,
except that the amount of cerium nitrate was changed to 200 g.
EXAMPLE 3
With an aqueous rhodium chloride solution containing 0.5 g of
rhodium, 5 g of alumina (.gamma.-Al.sub.2 O.sub.3) having a
specific surface area of 90 m.sup.2 /g was impregnated. Then, the
impregnated alumina was thoroughly dried and subsequently calcined
at 500.degree. C. for one hour.
Then, 200g of alumina (.gamma.-a1.sub.2 O.sub.3) having a specific
surface area of 155 m.sup.2 /g was impregnated with an aqueous
platinum chloride solution containing 2.5 g of platinum, to obtain
platinum-containing alumina. In a ball mill, the two powders
obtained as described above and 100 g of cerium compound obtained
in the same manner as in Example 1 were wet pulverized for 20
hours, to obtain an aqueous slurry. Monolithic carriers of
cordierite (33 mm in inside diameter and 76 mm in length)
possessing about 400 cells per square inch of cross section were
immersed in the aqueous slurry, removed from the slurry, blown with
compressed air to remove excess slurry from the cells, and then
dried, to obtain a catalyst 3. The coating layer of the catalyst
was photographed in 3,000 magnifications at 300 randomly selected
points and analyzed for distribution of platinum and rhodium by
electron probe micro analysis (EPMA), to find that
rhodium-containing alumina was dispersed in the form of particles
having an average diamter of 3 .mu.m and absolutely no platinum was
aggregated in the form of particle. By the fluorescent X-ray test,
the catalyst 3 was found to contain 0.081 g of platinum, 0.016 g of
rhodium, and 0.90 g of CeO.sub.2 per catalyst.
EXAMPLE 4
Alumina containing rhodium and platinum was obtained by
impregnating 10 g of alumina (.gamma.-A1.sub.2 O.sub.3) having a
specific surface area of 155 m.sup.2 /g with a mixed solution of
rhodium chloride and platinic chloride containing 0.5 g of rhodium
and 2.5 g of platinum, drying the impregnated alumina, and then
calcining the dried alumina at 500.degree. C. for one hour. In a
ball mill, the powder thus obtained 80 g of cerium compound
obtained in the same manner as in Example 2, and 210 g of
.gamma.-alumina were wet pulverized to obtain an aqueous slurry.
Thereafter, Catalyst 4 was obtained by following the procedure of
Example 3. By the EPMA analysis, the coating layer of this catalyst
was found to contain the platinum and rhodium-containing alumina as
dispersed in the form of particles having an average diameter of 5
.mu.m. The amount of the catalyst carried was as shown in Table
1.
Control 4
An aqueous solution of rhodium chloride and platinic chloride
containing 0.5 g of rhodium and 2.5 g of platinum was diluted with
purified water. Then, 220 g of .gamma.-alumina having a specific
surface area of 155 m.sup.2 /g was impregnated with the diluted
aqueous solution, dried, and then calcined at 500.degree. C. for
one hour. In a ball mill, the powder thus obtained and 80 g of
commercially available cerium oxide were wet pulverized, and
treated by following the procedure of Example 3, to obtain catalyst
d. When the coating layer of this catalyst was tested by EPMA,
neither platinum nor rhodium was detected in the form of particles
exceeding 0.5 .mu.m in diameter. The amount of the catalyst carried
was as shown in Table 1.
Control 5
Platinum-containing alumina was obtained by impregnating 100 g of
.gamma.-alumina with an aqueous solution of platinic chloride
containing 2.5 g of platinum, and then drying and calcining the
impregnated .gamma.-alumina. In a ball mill, the powder thus
obtained and 80 g of commercially available cerium oxide were wet
pulverized, to obtain an aqueous slurry. Then, 120 g of
.gamma.-alumina was impregnated with an aqueous solution of rhodium
chloride containing 0.5 g of rhodium and the resultant impregnated
.gamma. -alumina was pulverized in a ball mill, to obtain an
aqueous slurry. The rhodium in the aqueous slurry was fixed by
passing hydrogen sulfide gas through the rhodium-containing aqueous
slurry.
Monolithic carriers were wash coated with the mixture of the two
slurries obtained as described above. The coated carriers were
dried and calcined at 400.degree. C. for one hour to obtain a
finished catalyst e. When the coating layer of this catalyst was
tested by EPMA, neither platinum nor rhodium was found to be
contained in the form of particles exceeding 0.5 .mu.m in diameter.
The amount of catalyst carried was as shown in Table 1.
EXAMPLE 5
With an aqueous solution of rhodium nitrate containing 0.3 g of
rhodium, 5 g of .delta.-alumina having a specific surface area of
90 m.sup.2 /g was impregnated. The impregnated .delta.-alumina was
thoroughly dried and then calcined at 500.degree. C. for one hour,
to obtain rhodium-containing alumina. Then, 100 g of cerium
compound obtained in Example 2 was impregnated with an aqueous
solution of palladium nitrate containing 3.0 g of palladium and
then dried and calcined at 500.degree. C. for one hour.
An aqueous slurry was prepared by wet pulverizing the two powders
obtained as described above and 200 g of .delta.-alumina and then
processed by following the procedure of Example 3, to obtain
catalyst 5. When the coating layer of this catalyst was tested by
EPMA, the rhodium-containing alumina was found to be dispersed in
the form of particles having an average diameter of 6 .mu.m. The
amount of catalyst carried was as shown in Table 1.
Control 6
In a ball mill, 300 g of ceria-containing alumina obtained in the
same manner as in Control 1 was wet pulverized to prepare an
aqueous slurry and then processed by following the procedure of
Example 3 to obtain wash coated monolithic carriers. Then, the
carriers were calcined at 500.degree. C. for one hour, impregnated
with an aqueous solution of palladium chloride and rhodium
chloride, dried, and then calcined at 500.degree. C. to obtain
catalyst f. When the coating layer of the catalyst as tested by
EPMA, neither palladium nor rhodium was detected in the form of
particles exceeding 0.5 .mu.m in diameter. The amount of catalyst
carried was as shown catalyst Table 1.
TABLE 1 ______________________________________ Example No. Catalyst
No. Pt Pd Rh CeO.sub.2 ______________________________________
Example 3 3 0.081 -- 0.016 0.90 Example 4 4 0.080 -- 0.016 1.15
Control 4 d 0.082 -- 0.016 2.60 Control 5 e 0.081 -- 0.016 2.55
Example 5 5 -- 0.096 0.010 1.43 Control 6 f -- 0.102 0.010 2.73
(g/pc) ______________________________________
EXAMPLE 6
The samples obtained in Examples 1 and 2 Controls 1 to 3 and the
same samples further calcined at 1,000.degree. C. for 10 hours were
subjected to X-ray diffraction. The peaks consequently obtained are
shown in Table 2.
Separately, the samples of Example 1 and Control 1 and commercially
available ceria (having a specific surface area of 70 m.sup.2 /g)
calcined at 1,000.degree. C. for 10 hours for comparison were
tested for capacity for oxygen storage. The results were as shown
in Table 3. This test was carried out with a conventional flow type
apparatus by reducing a sample in hydrogen at 500.degree. C. and
then feeding oxygen pulses to the sample under continued flow of
helium thereby determining the amount of oxygen consumed by the
sample. From the results of Table 2 and Table 3, it is noted that
the ceria compound obtained in accordance with this invention does
not induce growth of ceria crystals but retains a highly desirable
capacity for oxygen storage even after exposure to an oxidizing
atmosphere at the elevated temperature of 1,000.degree. C.
TABLE 2 ______________________________________ CeO.sub.2 One hour'
Ten hour' Sample content (% calcination calcination No. by weight)
at 500.degree. C. at 1,000.degree. C.
______________________________________ 1 29 ZrO.sub.2 (monoclinic)
ZrO.sub.2 (tetragonal) 2 44 ZrO.sub.2 (monoclinic) ZrO.sub.2
(tetragonal) a 29 Al.sub.2 O.sub.3 (.gamma.), CeO.sub.2 Al.sub.2
O.sub.3 (.delta., .theta., .alpha.), CeO.sub.2 b 29 ZrO.sub.2
(monoclinic), ZrO.sub.2 (monoclinic), CeO.sub.2 CeO.sub.2 c 61
ZrO.sub.2 (monoclinic), ZrO.sub.2 (tetragonal), CeO.sub.2 CeO.sub.2
______________________________________
TABLE 3 ______________________________________ Sample No. Oxygen
consumption (mol) per mol of Ce
______________________________________ 1 9.8 .times. 10.sup.-2 a
0.1 .times. 10.sup.-2 Commercially 0.2 .times. 10.sup.-2 available
ceria ______________________________________
EXAMPLE 7
The catalysts, Nos. 3 to 5, and the catalysts, Nos. d to f, for
comparison were aged in an electric oven and then tested for
catalyic performance. The aging in the electric oven was carried
out under very harsh conditions of oxidizing atmosphere at an
elevated temperature, i.e. in the air at 900.degree. C., for 20
hours.
The evaluation of the catalytic performance was effected by the use
of an electronically controlled engine 4 cylinders 1,800 cc), with
the catalyst inlet gas temperature varied continuously with a heat
exchanger from 200.degree. to 450.degree. C., to find rations of
removal of CO, HC, NO.sub.x. In this case, the engine was operated
with the space velocity (S.V.) fixed at 90,000 hr.sup.-1, the
average air/fuel ratio (A/F) fixed at 14.6, and the vibration at
.+-.0.5 A/F 1 Hz. The catalyst inlet gas temperature at which the
ratios of removal of CO, HC, and NO.sub.x reached 50% (T50) and the
ratio of purification (%) at 450.degree. C. were as shown in Table
4.
The catalysts which had undergone the durability test in the engine
were similarly tested for catalytic performance. In the durability
test, an electronically controlled engine (8 cylinders 4,400 cc)
was operated in the mode of 60 seconds of steady driving and 6
seconds of decelerated driving (during which period the fuel was
cut and the catalyst was exposed to an oxidizing atmosphere at
elevated temperature). Thus, the catalyst was aged for 50 hours
under conditions such that the catalyst temperature would reach
800.degree. C. in the steady driving.
The evaluation of the sample for catalytic performance after the
durable test with the engine was carried out in the same manner as
described above. The results were as shown in Table 5. It is
clearly noted from the results of Tables 4 and 5 that the catalyst
disclosed by the present invention experiences sparing
deterioration and retains highly desirable durability not merely
under the normal engine driving conditions but also under such
harsh conditions as encountered in an oxidizing atmosphere at an
elevated temperature.
TABLE 4 ______________________________________ Temperature for 50%
Ratio of purification (%) conversion (.degree.C.) at 450.degree. C.
Catalyst No. CO HC NO CO HC NO
______________________________________ 3 372 375 369 85 83 87 4 365
371 360 88 87 88 d 426 431 422 69 65 71 e 403 410 401 72 70 72 5
355 358 356 97 98 96 f 394 397 399 83 85 82
______________________________________
TABLE 5 ______________________________________ Temperature for 50%
Ratio of purification (%) conversion (.degree.C.) at 450.degree. C.
Catalyst No. CO HC NO CO HC NO
______________________________________ 3 341 345 337 92 87 91 4 337
340 335 95 92 93 d 389 393 382 80 77 81 e 372 379 370 83 78 83 5
368 370 368 90 92 88 f 401 405 407 77 81 72
______________________________________
EXAMPLE 8
A zirconia-stabilized cerium oxide (hereinafter referred to as
"Ce-ZrO.sub.2 ") powder containing 26% by weight of ceria was
prepared by mixing an aqueous solution of cerium nitrate and
zirconia having a specific surface area of 82 m.sup.2 /g, drying
the resultant mixture, and then calcining the dry mixture in the
air at 500.degree. C. for two hours.
Then, a mixed powder comprising 20 g of the Ce--ZrO.sub.2 and 80 g
of activated alumina having a specific surface area of 100 m.sup.2
/g was impregnated with an aqueous rhodium nitrate solution
containing 0.1 g of rhodium. The impregnated powder was dried and
then calcined in the air at 500.degree. C. for two hours, to
prepare a catalyst composition.
This catalyst composition was converted into a slurry. Cylindrical
monolithic carriers of cordierite 33 mm in outside diameter and 76
mm in length possessing about 400 gas-passing cells per square inch
of cross section were coated with the catalyst composition slurry
and then dried at 130.degree. C. for three hours, to obtain a
finished catalyst carrying 6.5 g of the catalyst composition.
Part of the aforementioned Ce--ZrO.sub.2 powder containing 26% by
weight of ceria was calcined in the air at 900.degree. C. for ten
hours and subjected to X-ray diffraction. The X-ray diffraction
diagram showed only a peak for the tetragonal crystal of zirconia
as illustrated in FIG. 1. The calcined Ce--ZrO.sub.2 powder was
found to possess a specific surface area of 42 m.sup.2 /g.
EXAMPLE 9
A finished catalyst was obtained by following the procedure of
Example 8, except that a Ce--ZrO.sub.2 powder containing 13% by
weight of ceria was used and rhodium was carried on a mixed powder
comprising 50 g of the Ce--ZrO.sub.2 powder and 50 g of activated
alumina
Control 7
A catalyst composition was prepared by impregnating 80 g of
activated alumina having a specific surface area of 100 m.sup.2 /g
with a mixed aqueous solution containing 5.2 g of cerium nitrate as
ceria, 14.8 g of zirconium nitrate as zirconia, and 0.1 g of
rhodium nitrate as rhodium, drying the impregnated activated
alumina, and calcing the dry alumina in the air at 500.degree.
C.
This catalyst composition was converted into a slurry. In the same
manner as in Example 8, monolithic carriers were coated with the
slurry to obtain a finished catalyst carrying 6.5 g of the catalyst
composition.
Control 8
A mixed powder comprising of 10 g of ceria powder having a specific
surface area of 62 m.sup.2 /g, 60 g of activated alumina powder
having a specific surface area of 100 m.sup.2 /g, and 30 g of
zirconia powder having a specific surface area of 82 m.sup.2 /g was
impregnated with an aqueous solution containing 0.1 g of rhodium
nitrate as rhodium. The impregnated powder was dried and then
calcined at 500.degree. C., to prepare a catalyst composition. From
this catalyst composition, a finished catalyst carrying 6.5 g of
the catalyst composition was obtained by following the procedure of
Example 8.
Control 9
A finished catalyst was obtained by following the procedure of
Example 8, except that a zirconia powder having a specific surface
area of 38 m.sup.2 /g was used instead.
Part of the Ce--ZrO.sub.2 powder obtained herein was calcined in
the air at 900.degree. C. for ten hours and subjected to X-ray
diffraction. The X-ray diffraction diagram showed a peak for the
monoclinic crystal of zirconia and a peak for ceria as illustrated
in FIG. 2. The calcined powder was found to have a specific surface
area of 9.8 m.sup.2 /g.
Control 10
A finished catalyst was obtained by following the procedure of
Example 8, except that a Ce--ZrO.sub.2 powder containing 7% by
weight of ceria was used and rhodium was carried on a mixed powder
comprising 80 g of the Ce--ZrO.sub.2 powder and 20 g of activated
alumina
Part of the Ce--ZrO.sub.2 powder containing 7% by weight of ceria
was calcined in the air at 900.degree. C. for ten hours and
subjected to X-ray diffraction. The X-ray diffraction diagram
showed a main peak .for the monoclinic crystal of zirconia and a
peak for the tetragonal crystal of zirconia as illustrated in FIG.
3. The calcined powder was found to possess a specific surface area
of 28 m.sup.2 /g.
Control 11
A finished catalyst was obtained by following the procedure of
Example 8, except that a Ce--ZrO.sub.2 powder containing 85% by
weight of ceria was used instead and rhodium was carried on a mixed
powder comprising 15 g of the Ce--ZrO.sub.2 powder and 85 g of
activated alumina.
Part of the Ce--ZrO.sub.2 powder containing 85% by weight of ceria
was calcined in the air at 900.degree. C. for ten hours and then
subjected to X-ray diffraction. The X-ray diffraction diagram
showed a peak for ceria and partly a peak for the tetragonal
crystal of zirconia as illustrated in FIG. 4. The calcined powder
was found to possess a specific surface area of 3.1 m.sup.2 /g.
EXAMPLE 10
A Ce--ZrO.sub.2 powder containing 28% by weight of ceria based on
zirconia was prepared by mixing an aqueous solution of cerium
nitrate and zirconium hydroxide having a specific surface area of
98 m.sup.2 /g, drying the resultant mixture, and calcining the dry
mixture in the air at 500.degree. C. for two hours.
Then, a mixed powder comprising of 25 g of the Ce--ZrO.sub.2 powder
mentioned above and 115 g of a activated alumina containing 3% by
weight of ceria and 1% by weight of iron oxide and having a
specific surface area of 123 m.sup.2 /g was impregnated with a
mixed aqueous solution of containing 0.2 g of rhodium nitrate as
rhodium and 1 g of palladium nitrate as palladium. The impregnated
powder was dried and then calcined in the air at 500.degree. C. to
prepare a catalyst composition
This catalyst composition was converted into a slurry. In the same
manner as in Example 8, monolithic carriers were coated with the
slurry to obtain a finished catalyst carrying 9.2 g of the catalyst
composition.
EXAMPLE 11
A Ce--ZrO.sub.2 powder containing 23% by weight of ceria was
prepared by treating a mixed aqueous solution of cerium nitrate and
zirconium nitrate with aqua ammonia thereby inducing
coprecipitation of the salts, removing the coprecipitate, drying
and calcining in the air at 500.degree. C. for two hours. This
Ce--ZrO.sub.2 powder was found to have a specific surface area of
91 m.sup.2 /g.
Then, a finished catalyst carrying 9.2 g of a catalyst composition
was obtained by following the procedure of Example 10, except that
the Ce--ZrO.sub.2 powder prepared by the coprecipitation method was
used instead.
EXAMPLE 12
A finished catalyst was obtained by following the procedure of
Example 10, except that amorphous zirconia having a specific
surface area of 82 m.sup.2 /g was used as the zirconia source for
the Ce--ZrO.sub.2 powder
Control 12
A finished catalyst was obtained by following the procedure of
Example 10, except that a zirconia powder having a specific surface
area of 42 m.sup.2 /g was impregnated with cerium nitrate and a
Ce--ZrO.sub.2 powder containing 28% by weight of ceria based on
zirconia was prepared instead.
Control 13
A catalyst composition was prepared by mixing a mixed powder
comprising 115 g of activated alumina containing 5% by weight of
ceria and 1% by weight of iron oxide and having a specific surface
area of 123 m.sup.2 /g and 18 g of amorphous zirconia powder having
a specific surface area of 82 m.sup.2 /g and 7g of cerium nitrate
as ceria, 0.2 g of rhodium nitrate as rhodium, and 1 g of palladium
nitrate as palladium, drying the resultant mixture, and calcining
the dry mixture in the air at 500.degree. C. for two hours.
From this catalyst composition, a finished catalyst carrying 9.2 g
of the catalyst composition was obtained by coating monolithic
carriers with the catalyst composition in the same manner as in
Example 10.
The amounts of catalyst components carried per liter of each of the
finished catalysts of Examples 8 to 12 and Controls 7 to 13 were as
shown in Table 6.
TABLE 6 ______________________________________ Amount of catalyst
components carried per liter of finished catalyst (g/liter) Total
amount Catalyst Pd Rh CeO.sub.2 ZrO.sub.2 Fe.sub.2 O.sub.3 Al.sub.2
O.sub.3 carried ______________________________________ Example 8 --
0.1 5.2 14.8 -- 80 100.1 Example 9 -- 0.1 6.7 43.3 -- 50 100.1
Control 7 -- 0.1 5.2 14.8 -- 80 100.1 Control 8 -- 0.1 10 30 -- 60
100.1 Control 9 -- 0.1 5.2 14.8 -- 80 100.1 Control 10 -- 0.1 5.5
74.5 -- 20 100.1 Control 11 -- 0.1 12.7 2.3 -- 85 100.1 Example 10
1.0 0.2 12.6 17.8 1.8 107.6 141.0 Example 11 1.0 0.2 11.5 18.9 1.8
107.6 141.0 Example 12 1.0 0.2 12.6 17.8 1.8 107.6 141.0 Control 12
1.0 0.2 12.6 17.8 1.8 107.6 141.0 Control 13 1.0 0.2 12.6 17.8 1.8
107.6 141.0 ______________________________________
EXAMPLE 13
The catalyst of Example 8 to 12 and the catalysts of Controls 7 to
13 were tested for catalytic activity after a durability test in an
engine.
The durability test was formed by use of a commercially available
electronically controlled engine (8 cylinders 4,400 cc), with a
multi-converter filled with a sample catalyst and connected to the
exhaust system of the engine. The engine was operated by a mode of
60 seconds' steady driving and 6 seconds' decelerated driving
(during which period the fuel was cut and the catalyst was exposed
to harsh conditions of an oxidizing atmosphere at elevated
temperature), to age the catalyst for 50 hours under conditions
such that the catalyst inlet gas temperature during the steady
driving would reach 800.degree. C.
The evaluation of the aged catalyst for catalytic performance was
carried out by use of a commercially available electronically
controlled engine (4 cylinders, 1,800 cc), with a multi-converter
filled with a sample catalyst and connected to the exhaust system
of the engine. The performance of the catalyst was evaluated under
the conditions of catalyst inlet gas temperature of 450.degree. C
and a space velocity of 90,000 hr.sup.-1. In this case, a 1-Hz sine
wave form signal from an external oscillator was fed into a control
unit of the engine to effect continuous change of the average
air/fuel ratio, with the air/fuel ratio (A/F) fluctuated within the
range of .+-.0.5 A/F and 1 Hz. In the meantime, the gas sampled at
the inlet and outlet of the catalyst was analyzed to find the
ratios of removal of CO, HC, and NO.sub.x for the average air/fuel
ratio, A/F, in the range of 15.1 to 14.1.
A three way characteristic curve was obtained by plotting on a
graph the data of the ratio of removal of CO, HC, and NO.sub.x
obtained as described above vs. the inlet air/fuel ratio. The ratio
of purification at the intersection of the curves of ratios of
removal of CO and NO.sub.x (hereinafter referred to as "crossover
point") and the ratio of removal of HC at the A/F value of that
crossover point were found from the three way characteristic curve
to be used as criteria for the evaluation of the three way
performance of the catalyst. C.
The evaluation of the catalyst with respect to the purification
capacity at low temperatures was carried out by operating the
engine with the average air/fuel ratio, A/F, fixed at 14.6 and the
air/fuel ratio fluctuated within the range of .+-. 0.5 A/F and 1
Hz, continuously changing the catalyst inlet gas temperature from
200.degree. to 500.degree. C. by means of a heat-exchanger disposed
before the catalyst converter in the exhaust system of the engine,
simultaneously analyzing the gas sampled at the inlet and outlet of
the catalyst, and determining the ratios of removal of CO, HC, and
NO.sub.x.
The data consequently obtained on the ratios of removal of CO, HC,
and NO.sub.x vs. the catalyst inlet gas temperature were plotted on
a graph. The catalyst inlet gas temperature at which the ratio of
purification reached 50% (R.sub.50) was found from the curve of the
graph and used as a criterion for the evaluation of the catalyst's
purification ability at low temperatures.
The results of the evaluation of catalyst performance obtained by
the method described above were as shown in Table 7.
TABLE 7
__________________________________________________________________________
Results of evaluation of catalyst performance after durability test
with engine Three way performance Purification ability at low
temperature Crossover point Ratio of Ratio of Ratio of Ratios of
removal Ratio of removal removal of CO removal of HC removal of NOx
Catalyst of CO and NO (%) of HC (%) T 50 (.degree.C.) T 50
(.degree.C.) T 50 (.degree.C.)
__________________________________________________________________________
Example 8 96.5 92.6 345 349 343 Example 9 95.8 91.6 351 357 349
Control 7 50.8 51.2 428 425 436 Control 8 60.5 58.6 415 416 424
Control 9 45.6 48.2 424 418 415 Control 10 72.5 65.8 408 412 401
Control 11 61.8 57.4 414 417 409 Example 10 98.1 95.3 342 341 330
Example 11 98.8 98.2 328 334 319 Example 12 99.2 96.8 336 343 321
Control 12 82.8 83.5 421 427 409 Control 13 85.6 82.1 404 406 409
__________________________________________________________________________
It is clearly noted from Table 7 that the catalysts of this
invention containing a ceria-zirconia compound having a
platinum-family metal and ceria incorporated in the form of a solid
solution in zirconia in a concentration in the range of 10 to 50%
by weight as contemplated by the present invention exhibit highly
satisfactory catalyst performance as compared with the conventional
catalyst using ceria and zirconia.
From the results described above, it is noted that the catalyst
offered by the present invention possesses an outstanding
purification ability and, at the same time, retains high durability
even under such harsh conditions as experienced in an oxidizing
atmosphere at an elevated temperature.
EXAMPLE 14
With a solution in 100 ml of water of 50 g of cerium nitrate
[Ce(NO.sub.3).sub.3 6H.sub.2 O], 50 g of minute particles of
zirconia having a specific surface area of 97 m.sup.2 /g were
impregnated. The impregnated particles were thoroughly dried and
then calcined at 500.degree. C. for one hour. (The product was
labelled as Sample No. 6)
EXAMPLE 15
With a solution in 100 ml of water of 50 g of cerium nitrate
[Ce(No.sub.3).sub.3 6H.sub.2 O] and 1.7 g of yttrium nitrate
[Y(No.sub.3).sub.3 6H.sub.2 O], 50 g of the same minute particles
of zirconia as in Example 14 were impregnated. The impregnated
particles were thoroughly dried and then calcined at 500.degree. C.
for one hour. (The product was labelled as Sample No. 7.)
EXAMPLE 16
With a solution in 100 ml of water of 50 g of cerium nitrate
[Ce(NO.sub.3).sub.3 6H.sub. 2 O] and 3.4 g of yttrium nitrate
[Y(NO.sub.3).sub.3 6H.sub.2 O)], 50 g of minute particles of
zirconia having a specific surface area of 97 m.sup.2 /g were
impregnated. The impregnated particles were thoroughly dried and
then calcined at 500.degree. C. for one hour. (The product was
labelled as Sample No. 8.)
EXAMPLE 17
Sample No. 9 was obtained by following the procedure of Example 16,
except that the amount of yttrium nitrate was changed to 6.8 g.
EXAMPLE 18
Sample No. 10 was obtained by following the procedure of Example
16, except that 8.4 g of calcium nitrate [Ca(NO.sub.3).sub.2
4H.sub.2 O] was used in place of yttrium nitrate.
EXAMPLE 19
With solution in 100 ml of water of 50 g of cerium nitrate, 3.4 g
of yttrium nitrate and 8.4 g of calcium nitrate, 50 g of the same
minute particles of zirconia as in Example 14 was impregnated. The
impregnated particles were thoroughly dried and then calcined at
500.degree. C. for 1 hour. (The product was labelled as Sample No.
11)
Control 14
Sample g was obtained by following the procedure of Example 16,
except that zirconia having a specific surface area of 40 m.sup.2
/g was used instead.
Control 15
Sample h was obtained by following the procedure of Example 17,
except that the addition of cerium nitrate was omitted
Control 16
Sample i was obtained by following the procedure of Example 16,
except that 50 g of .gamma.-alumina having a specific surface area
of 150 m.sup.2 /g was used in place of zirconia.
EXAMPLE 20
[1] The cerium oxide-containing powders of Samples No. 6 to 11 and
Samples g to i obtained respectively in Examples 14 to 19 and
Controls 14 to 16 and a commercially available ceria of a large
specific surface area (85 m.sup.2 /g) were treated in an electric
oven at 1,000.degree. C. for ten hours and subjected to X-ray
diffraction and tested for BET surface area. The results were as
shown in Table 8.
The cerium oxide-containing powders cf Example 14 to 19 showed no
growth of CeO.sub.2 even after the treatment at the elevated
temperature and further exhibited stable BET surface area due to
addition of yttrium and/or calcium.
[2] The samples Nos. 8 and 9, the sample i, and commercially
available cerium oxide were tested for XPS. The results were as
shown in FIG. 5. It is noted from FIG. 5 that the cerium oxides
obtained in the working examples of this invention show a shift
from the ordinary 3d peak of Ce toward the lower bond energy side.
The fact that the bond energy is shifted toward the lower energy
side means that the oxidation number of Ce is increased. It is,
therefore, logical to infer that the addition of yttrium results in
the formation of a Ce seed of an enhanced state of oxidation.
TABLE 8
__________________________________________________________________________
Results of X-ray diffraction test (1,000.degree. C. .times. 10
hours' calcination) Component and crystalline system B.E.Tum Sample
Composition X-ray diffraction peak SA(m.sup.2 /g)
__________________________________________________________________________
6 ZrO.sub.2 50: CeO.sub.2 20 ZrO.sub.2 (tetragonal) 12.6 7
ZrO.sub.2 50: CeO.sub.2 20: Y.sub.2 O.sub.3 1 ZrO.sub.2
(tetragonal) 17.9 8 ZrO.sub.2 50: CeO.sub.2 20: Y.sub.2 O.sub.3 2
ZrO.sub.2 (tetragonal) 23.1 9 ZrO.sub.2 50: CeO.sub.2 20: Y.sub.2
O.sub.3 4 ZrO.sub.2 (tetragonal) 20.8 10 ZrO.sub.2 50: CeO.sub.2
20: --: CaO 2 ZrO.sub.2 (tetragonal) 18.4 11 ZrO.sub.2 50:
CeO.sub.2 20: Y.sub.2 O.sub.3 2: CaO.sub.2 ZrO.sub.2 (tetragonal)
22.5 g ZrO.sub.2 50: CeO.sub.2 20: Y.sub.2 O.sub.3 2 ZrO.sub.2
(monoclinic) CeO.sub.2 5.4 h ZrO.sub.2 50: --: Y.sub.2 O.sub.3 4
ZrO.sub.2 (tetragonal) 10.3 i Al.sub.2 O.sub.3 50: CeO.sub.2 20:
Y.sub.2 O.sub.3 , Al.sub.2 O.sub.3, CeO.sub.2 54.9 Commercially --
CeO.sub.2 3.4 available CeO.sub.2
__________________________________________________________________________
EXAMPLE 21
An alumina powder containing platinum and rhodium was obtained by
impregnating 200 g of activated alumina having a specific surface
area of 150 m.sup.2 /g with a mixture of an aqueous solution of
dinitrodianmine platinic nitrate containing 1.5 g of platinum and
an aqueous solution of rhodium nitrate containing 0.3 g of rhodium,
thoroughly drying the impregnated activated alumina, and calcining
the dry alumina in the air at 400.degree. C. for two hours.
Then, in a ball mill, the platinum-family metal containing alumina
obtained as described above and 100 g of the cerium
oxide-containing powder obtained in Example 14 were blended with
water and nitric acid to prepare an aqueous slurry.
A finished catalyst No. 12 was obtained by coating monolithic
carriers (33 mm in diameter and 110 mm in length) of cordielite
with the aqueous slurry and then drying the coated carriers at
130.degree. C. for three hours. The amount of the slurry coating of
the finished catalyst was 150 g/liter.
EXAMPLES 22 TO 26 AND CONTROLS 17 to 19
Finished catalyst Nos. 13 to 17 and No. j to 1 were obtained by
following the procedure of Example 21, except that the cerium
oxide-containing powder obtained in Examples 15 to 19 and Controls
14 to 16 were used in such amounts as shown in Table 8.
Control 20
A finished catalyst No. m was obtained by following the procedure
of Example 21, except that 100 g of a commercially available ceria
(85 m.sup.2 /g) was used instead.
EXAMPLE 20
The catalysts Nos. 12-17, and the catalysts Nos. j-m of Controls
were sujbected to the durability test with an engine and then
tested for catalytic performance.
The durability test was performed by use of an electronically
controlled engine (8 cylinders, 4,400 cc). The engine was operated
by a mode of 60 seconds' steady driving and 6 seconds' decelerated
driving (during which period the fuel was cut and the sample
catalyst was exposed to an oxidizing atmosphere at an elevated
temperature) and the sample catalyst as aged for 50 hours under the
conditions such that the catalyst temperature reached 900.degree.
C. in the steady driving.
The evaluation of the catalyst was performed by use of an
electronically controlled engine by continuously changing the
temperature from 200.degree. C. to 450.degree. C. by means of a
heat-exchanger and simultaneously determining the catalyst inlet
gas temperature for the ratios of removal of CO, HC, and NO.sub.x
to reach 50% (T.sub.50) and the ratio of removal of the components
at 450.degree. C. The results were as shown in Table 9.
It is clearly noted from Table 9 that the catalysts according to
the present invention retain highly desirable durability even under
harsh conditions as experienced in an oxidizing atmosphere at
elevated temperatures owing to the stabilization of cerium
oxide.
TABLE 9 ______________________________________ 50% conversion
temperature Ratio of purification at Catalyst (.degree.C.)
450.degree. C. (%) No. CO HC NO CO HC NO
______________________________________ 12 358 363 356 88 85 89 13
341 348 338 92 86 93 14 337 341 332 94 86 94 15 329 334 322 95 90
94 16 334 340 329 94 86 94 17 327 331 319 95 90 94 j 389 394 382 79
80 81 k 365 367 360 68 75 69 l 397 403 395 63 72 66 m 403 412 401
72 68 73 ______________________________________ Air/fuel ratio of
exhaust gas from engine, A/F, = 14.6 .+-. 0.5, 1 Hz. Temperature
increasing speed 10.degree. C./min.
* * * * *